Method for reporting state of relay operation performed by terminal in radio communication system and terminal using same

ABSTRACT

Provided are a method for reporting a state of a relay operation performed by a terminal in a radio communication system and a terminal using the method. The method evaluates an event related to a state of a relay operation of the terminal and, when the event is satisfied, transmits a report for the state of the relay operation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the National Phase of PCT International ApplicationNo. PCT/KR2015/000957, filed on Jan. 29, 2015, which claims priorityunder 35 U.S.C. 119(e) to U.S. Provisional Application No. 61/932,794,filed on Jan. 29, 2014, all of which are hereby expressly incorporatedby reference into the present application.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to wireless communication, and moreparticularly to a method for reporting a state of a relay functionperformed by a terminal in a radio communication system and a terminalusing the same.

Related Art

In an International Telecommunication Union Radio communication sector(ITU-R), a standardization of International Mobile Telecommunication(IMT)-Advanced being a next mobile communication system after a thirdgeneration has been performed. The IMT-Advanced is aimed at supportingan Internet Protocol (IP) based multi-media service with a datatransmission rate of 1 Gbps in a stop and low speed moving state and adata transmission rate of 1 Gbps in a high speed moving state.

A 3rd Generation Partnership Project (3GPP) is preparing LTE-Advanced(LTE-A) being an improved one of Long Term Evolution (LTE) based on anOFDMA (Orthogonal Frequency Division Multiple Access)/SC-FDMA (SingleCarrier-Frequency Division Multiple Access) transmission scheme as asystem standard satisfying requirements of IMT-Advanced. The LTE-A isone important candidate for IMT-Advanced.

In recent years, there is growing interest in a Device-to-Device (D2D)technology performing direct communication between devices. Inparticular, the D2D is attracting attention as a communicationtechnology for a public safety network. A commercial communicationnetwork has been rapidly changed to the LTE but a current public safetynetwork is based on a 2G technology in a collision problem and a costside with an existing communication standard. Request for the technologyclearance and an improved service induces an effort to improve thepublic safety network.

The public safety network has high service requirements (reliability andsecurity) as compared with a commercial communication network. Inparticular, when coverage of cellular communication is insufficient oris not used, there is a need for direct signal transmission/receptionbetween devices, that is, an D2D operation.

The D2D operation may be signal transmission/reception between adjacentdevices to have various advantages. For example, a D2D terminal mayperform data communication with a high transmission rate and low delay.Further, the D2D operation may distribute traffic converged in a basestation. If the D2D terminal serves as a relay, the D2D terminal mayserve to extend coverage of a base station.

In a related art, a network controls a relay, whereas in a D2Doperation, UE may autonomously determine the start/stop/resumption ofthe provision of a relay functionality. That is, in the D2D operation,even such a scenario needs to be taken into consideration.

Furthermore, specific UE already functions as a relay or is expected tofunction as a relay. Another piece of UE adjacent to the specific UE mayfunction as a relay or attempt to function as a relay. If resources usedby the specific UE and another piece of UE overlap partially or fully,interference between them is caused, and use efficiency of radioresources is also deteriorated.

If UE supporting a D2D operation is able to provide the relayfunctionality, the aforementioned problem may be generated, and there isa need for a method and apparatus for solving such a problem.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method for reportinga relay operation status, which is performed by UE, in a wirelesscommunication system and UE using the method.

In an aspect, a method for reporting a relay operation status performedby user equipment (UE) in a wireless communication system is provided.The method comprising evaluating an event related to a relay operationstatus of the UE and transmitting a report on the relay operation statusif the event is satisfied.

In another aspect, a user equipment (UE) is provided. The UE comprises aradio frequency (RF) unit transmitting and receiving radio signals and aprocessor operating in conjunction with the RF unit, wherein theprocessor evaluates an event related to a relay operation status of theUE and transmits a report on the relay operation status if the event issatisfied.

The present invention may be applied to a case where UE supporting a D2Doperation provides a relay functionality. In accordance with the presentinvention, relay operation status information is transmitted so thatother pieces of UE are able to be aware of the relay operation status ofthe UE. As a result, any one of the UE and other pieces of UE does notprovide an unnecessarily redundant relay functionality, and resourceefficiency is also improved. Furthermore, UE can also send a relayoperation status report to a BS. The eNB can receive the relay operationstatus information from each piece of UE and give a proper command toeach piece of UE. Accordingly, a probability that UE providing a relayfunctionality may be subjected to interference from another piece of UEis reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system to which the presentinvention is applied.

FIG. 2 is a diagram showing a wireless protocol architecture for a userplane.

FIG. 3 is a diagram showing a wireless protocol architecture for acontrol plane.

FIG. 4 is a flowchart illustrating the operation of UE in the RRC idlestate.

FIG. 5 is a flowchart illustrating a procedure of establishing RRCconnection.

FIG. 6 is a flowchart illustrating an RRC connection reconfigurationprocedure.

FIG. 7 is a diagram illustrating an RRC connection re-establishmentprocedure.

FIG. 8 illustrates sub states where the terminal may have in an RRC_IDLEstate and a sub state transition process.

FIG. 9 illustrates a reference structure for a ProSe.

FIG. 10 illustrates arrangement examples of terminals performing ProSedirect communication and cell coverage.

FIG. 11 illustrates a user plane protocol stack for the ProSe directcommunication.

FIG. 12 illustrates a PC 5 interface for D2D discovery.

FIG. 13 illustrates an embodiment of a ProSe direct discovery procedure.

FIG. 14 illustrates another embodiment of a ProSe direct discoveryprocedure.

FIG. 15 shows a UE-NW relay.

FIG. 16 shows a UE-UE relay.

FIG. 17 shows the number of hops of a UE-NW relay.

FIG. 18 shows the number of hops of another UE-NW relay.

FIG. 19 shows a method for reporting a relay operation status, which isperformed by UE.

FIG. 20 shows a method for reporting a relay operation status accordingto an embodiment of the present invention.

FIG. 21 shows a method for reporting a relay operation status accordingto another embodiment of the present invention.

FIG. 22 is a modified example of the embodiment of FIG. 21.

FIG. 23 is a block diagram of UE in which the embodiment of the presentinvention is implemented.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows a wireless communication system to which the presentinvention is applied. The wireless communication system may also bereferred to as an evolved-UMTS terrestrial radio access network(E-UTRAN) or a long term evolution (LTE)/LTE-A system.

The E-UTRAN includes at least one base station (BS) 20 which provides acontrol plane and a user plane to a user equipment (UE) 10. The UE 10may be fixed or mobile, and may be referred to as another terminology,such as a mobile station (MS), a user terminal (UT), a subscriberstation (SS), a mobile terminal (MT), a wireless device, etc. The BS 20is generally a fixed station that communicates with the UE 10 and may bereferred to as another terminology, such as an evolved node-B (eNB), abase transceiver system (BTS), an access point, etc.

The BSs 20 are interconnected by means of an X2 interface. The BSs 20are also connected by means of an S1 interface to an evolved packet core(EPC) 30, more specifically, to a mobility management entity (MME)through S1-MME and to a serving gateway (S-GW) through S1-U.

The EPC 30 includes an MME, an S-GW, and a packet data network-gateway(P-GW). The MME has access information of the UE or capabilityinformation of the UE, and such information is generally used formobility management of the UE. The S-GW is a gateway having an E-UTRANas an end point. The P-GW is a gateway having a PDN as an end point.

Layers of a radio interface protocol between the UE and the network canbe classified into a first layer (L1), a second layer (L2), and a thirdlayer (L3) based on the lower three layers of the open systeminterconnection (OSI) model that is well-known in the communicationsystem. Among them, a physical (PHY) layer belonging to the first layerprovides an information transfer service by using a physical channel,and a radio resource control (RRC) layer belonging to the third layerserves to control a radio resource between the UE and the network. Forthis, the RRC layer exchanges an RRC message between the UE and the BS.

FIG. 2 is a diagram showing a wireless protocol architecture for a userplane. FIG. 3 is a diagram showing a wireless protocol architecture fora control plane. The user plane is a protocol stack for user datatransmission. The control plane is a protocol stack for control signaltransmission.

Referring to FIGS. 2 and 3, a PHY layer provides an upper layer with aninformation transfer service through a physical channel. The PHY layeris connected to a medium access control (MAC) layer which is an upperlayer of the PHY layer through a transport channel. Data is transferredbetween the MAC layer and the PHY layer through the transport channel.The transport channel is classified according to how and with whatcharacteristics data is transferred through a radio interface.

Data is moved between different PHY layers, that is, the PHY layers of atransmitter and a receiver, through a physical channel. The physicalchannel may be modulated according to an Orthogonal Frequency DivisionMultiplexing (OFDM) scheme, and use the time and frequency as radioresources.

The functions of the MAC layer include mapping between a logical channeland a transport channel and multiplexing and demultiplexing to atransport block that is provided through a physical channel on thetransport channel of a MAC Service Data Unit (SDU) that belongs to alogical channel. The MAC layer provides service to a Radio Link Control(RLC) layer through the logical channel.

The functions of the RLC layer include the concatenation, segmentation,and reassembly of an RLC SDU. In order to guarantee various types ofQuality of Service (QoS) required by a Radio Bearer (RB), the RLC layerprovides three types of operation mode: Transparent Mode (TM),Unacknowledged Mode (UM), and Acknowledged Mode (AM). AM RLC provideserror correction through an Automatic Repeat Request (ARQ).

The RRC layer is defined only on the control plane. The RRC layer isrelated to the configuration, reconfiguration, and release of radiobearers, and is responsible for control of logical channels, transportchannels, and PHY channels. An RB means a logical route that is providedby the first layer (PHY layer) and the second layers (MAC layer, the RLClayer, and the PDCP layer) in order to transfer data between UE and anetwork.

The function of a Packet Data Convergence Protocol (PDCP) layer on theuser plane includes the transfer of user data and header compression andciphering. The function of the PDCP layer on the user plane furtherincludes the transfer and encryption/integrity protection of controlplane data.

What an RB is configured means a procedure of defining thecharacteristics of a wireless protocol layer and channels in order toprovide specific service and configuring each detailed parameter andoperating method. An RB can be divided into two types of a Signaling RB(SRB) and a Data RB (DRB). The SRB is used as a passage through which anRRC message is transmitted on the control plane, and the DRB is used asa passage through which user data is transmitted on the user plane.

If RRC connection is established between the RRC layer of UE and the RRClayer of an E-UTRAN, the UE is in the RRC connected state. If not, theUE is in the RRC idle state.

A downlink transport channel through which data is transmitted from anetwork to UE includes a broadcast channel (BCH) through which systeminformation is transmitted and a downlink shared channel (SCH) throughwhich user traffic or control messages are transmitted. Traffic or acontrol message for downlink multicast or broadcast service may betransmitted through the downlink SCH, or may be transmitted through anadditional downlink multicast channel (MCH). Meanwhile, an uplinktransport channel through which data is transmitted from UE to a networkincludes a random access channel (RACH) through which an initial controlmessage is transmitted and an uplink shared channel (SCH) through whichuser traffic or control messages are transmitted.

Logical channels that are placed over the transport channel and that aremapped to the transport channel include a broadcast control channel(BCCH), a paging control channel (PCCH), a common control channel(CCCH), a multicast control channel (MCCH), and a multicast trafficchannel (MTCH).

The physical channel includes several OFDM symbols in the time domainand several subcarriers in the frequency domain. One subframe includes aplurality of OFDM symbols in the time domain. An RB is a resourcesallocation unit, and includes a plurality of OFDM symbols and aplurality of subcarriers. Furthermore, each subframe may use specificsubcarriers of specific OFDM symbols (e.g., the first OFDM symbol) ofthe corresponding subframe for a physical downlink control channel(PDCCH), that is, an L1/L2 control channel. A Transmission Time Interval(TTI) is a unit time for subframe transmission.

The RRC state of UE and an RRC connection method are described below.

The RRC state means whether or not the RRC layer of UE is logicallyconnected to the RRC layer of the E-UTRAN. A case where the RRC layer ofUE is logically connected to the RRC layer of the E-UTRAN is referred toas an RRC connected state. A case where the RRC layer of UE is notlogically connected to the RRC layer of the E-UTRAN is referred to as anRRC idle state. The E-UTRAN may check the existence of corresponding UEin the RRC connected state in each cell because the UE has RRCconnection, so the UE may be effectively controlled. In contrast, theE-UTRAN is unable to check UE in the RRC idle state, and a Core Network(CN) manages UE in the RRC idle state in each tracking area, that is,the unit of an area greater than a cell. That is, the existence ornon-existence of UE in the RRC idle state is checked only for each largearea. Accordingly, the UE needs to shift to the RRC connected state inorder to be provided with common mobile communication service, such asvoice or data.

When a user first powers UE, the UE first searches for a proper cell andremains in the RRC idle state in the corresponding cell. The UE in theRRC idle state establishes RRC connection with an E-UTRAN through an RRCconnection procedure when it is necessary to set up the RRC connection,and shifts to the RRC connected state. A case where UE in the RRC idlestate needs to set up RRC connection includes several cases. Forexample, the cases may include a need to send uplink data for a reason,such as a call attempt by a user, and to send a response message as aresponse to a paging message received from an E-UTRAN.

A Non-Access Stratum (NAS) layer placed over the RRC layer performsfunctions, such as session management and mobility management.

In the NAS layer, in order to manage the mobility of UE, two types ofstates: EPS Mobility Management-REGISTERED (EMM-REGISTERED) andEMM-DEREGISTERED are defined. The two states are applied to UE and theMME. UE is initially in the EMM-DEREGISTERED state. In order to access anetwork, the UE performs a procedure of registering it with thecorresponding network through an initial attach procedure. If the attachprocedure is successfully performed, the UE and the MME become theEMM-REGISTERED state.

In order to manage signaling connection between UE and the EPC, twotypes of states: an EPS Connection Management (ECM)-IDLE state and anECM-CONNECTED state are defined. The two states are applied to UE andthe MME. When the UE in the ECM-IDLE state establishes RRC connectionwith the E-UTRAN, the UE becomes the ECM-CONNECTED state. The MME in theECM-IDLE state becomes the ECM-CONNECTED state when it establishes S1connection with the E-UTRAN. When the UE is in the ECM-IDLE state, theE-UTRAN does not have information about the context of the UE.Accordingly, the UE in the ECM-IDLE state performs procedures related toUE-based mobility, such as cell selection or cell reselection, without aneed to receive a command from a network. In contrast, when the UE is inthe ECM-CONNECTED state, the mobility of the UE is managed in responseto a command from a network. If the location of the UE in the ECM-IDLEstate is different from a location known to the network, the UE informsthe network of its corresponding location through a tracking area updateprocedure.

System information is described below.

System information includes essential information that needs to be knownby UE in order for the UE to access a BS. Accordingly, the UE needs tohave received all pieces of system information before accessing the BS,and needs to always have the up-to-date system information. Furthermore,the BS periodically transmits the system information because the systeminformation is information that needs to be known by all UEs within onecell. The system information is divided into a Master Information Block(MIB) and a plurality of System Information Blocks (SIBs).

The MIB may include a limited number of parameters that are mostessential and most frequently transmitted when other information isrequired to be obtained from a cell. UE first searches for an MIB afterdownlink synchronization. The MIB may include information, such as anSFN that supports downlink channel bandwidth, a PHICH configuration, andsynchronization and operates as a timing criterion and an eNB transmitantenna configuration. The MIB may be transmitted on a broadcast channel(BCH) through broadcasting.

SystemInformationBlockType1 (SIB1) of included SIBs is included in a“SystemInformationBlockType1” message and transmitted. The remainingSIBs other than the SIB1 is included in a system information message andtransmitted. To map the SIBs to the system information message may beflexibly configured by a scheduling information list parameter includedin the SIB1. In this case, each of the SIBs is included in a singlesystem information message, and only SIBs having the same schedulingrequirement value (e.g. cycle) may be mapped to the same systeminformation message. Furthermore, a SystemInformationBlockType2 (SIB2)is always mapped to a system information message corresponding to thefirst entry within the system information message list of a schedulinginformation list. A plurality of system information messages may betransmitted within the same cycle. The SIB1 and all the systeminformation messages are transmitted on a DL-SCH.

In addition to broadcast transmission, in an E-UTRAN, the SIB1 may bededicated-signaled in the state in which it includes a parameterconfigured like an existing configured value. In this case, the SIB1 maybe included in an RRC connection reconfiguration message andtransmitted.

The SIB1 includes information related to UE cell access, and defines thescheduling of other SIBs. The SIB1 may include information related tothe PLMN identifiers of a network, tracking area code (TAC) and a cellID, a cell barring status indicative of whether a cell is a cell onwhich camp-on is possible, the lowest reception level required within acell which is used as cell reselection criterion, and the transmissiontime and cycle of other SIBs.

The SIB2 may include radio resource configuration information common toall pieces of UE. The SIB2 may include information related to an uplinkcarrier frequency and uplink channel bandwidth, an RACH configuration, apage configuration, an uplink power control configuration, a soundingreference signal configuration, a PUCCH configuration supportingACK/NACK transmission, and a PUSCH configuration.

UE may apply a procedure for obtaining system information and detectinga change of system information to a primary cell (PCell) only. In asecondary cell (SCell), when a corresponding SCell is added, an E-UTRANmay provide all of pieces of system information related to an RRCconnection state operation through dedicated signaling. When systeminformation related to a configured SCell is changed, an E-UTRAN mayrelease an SCell that is taken into consideration and subsequently addthe changed system information. This may be performed along with asingle RRC connection reconfiguration message. An E-UTRAN may configureparameter values different from a value broadcasted within an SCell thathas been taken into consideration through dedicated signaling.

UE needs to guarantee the validity of a specific type of systeminformation, and such system information is called required systeminformation. The required system information may be defined as follows.

-   -   If UE is an RRC idle state: The UE needs to be guaranteed so        that it has the valid versions of the MIB and the SIB1 in        addition to the SIB2 to SIBS. This may comply with the support        of a radio access technology (RAT) that is taken into        consideration.    -   If UE is an RRC connection state: The UE needs to be guaranteed        so that it has the valid versions of the MIB, the SIB 1, and the        SIB2.

In general, the validity of system information may be guaranteed up to amaximum of 3 hours after the system information is obtained.

In general, service that is provided to UE by a network may beclassified into three types as follows. Furthermore, the UE differentlyrecognizes the type of cell depending on what service may be provided tothe UE. In the following description, a service type is first described,and the type of cell is described.

1) Limited service: this service provides emergency calls and anEarthquake and Tsunami Warning System (ETWS), and may be provided by anacceptable cell.

2) Suitable service: this service means public service for common uses,and may be provided by a suitable cell (or a normal cell).

3) Operator service: this service means service for communicationnetwork operators. This cell may be used by only communication networkoperators, but may not be used by common users.

In relation to a service type provided by a cell, the type of cell maybe classified as follows.

1) An acceptable cell: this cell is a cell from which UE may be providedwith limited service. This cell is a cell that has not been barred froma viewpoint of corresponding UE and that satisfies the cell selectioncriterion of the UE.

2) A suitable cell: this cell is a cell from which UE may be providedwith suitable service. This cell satisfies the conditions of anacceptable cell and also satisfies additional conditions. The additionalconditions include that the suitable cell needs to belong to a PublicLand Mobile Network (PLMN) to which corresponding UE may access and thatthe suitable cell is a cell on which the execution of a tracking areaupdate procedure by the UE is not barred. If a corresponding cell is aCSG cell, the cell needs to be a cell to which UE may access as a memberof the CSG.

3) A barred cell: this cell is a cell that broadcasts informationindicative of a barred cell through system information.

4) A reserved cell: this cell is a cell that broadcasts informationindicative of a reserved cell through system information.

FIG. 4 is a flowchart illustrating the operation of UE in the RRC idlestate. FIG. 4 illustrates a procedure in which UE that is initiallypowered on experiences a cell selection procedure, registers it with anetwork, and then performs cell reselection if necessary.

Referring to FIG. 4, the UE selects Radio Access Technology (RAT) inwhich the UE communicates with a Public Land Mobile Network (PLMN), thatis, a network from which the UE is provided with service (S410).Information about the PLMN and the RAT may be selected by the user ofthe UE, and the information stored in a Universal Subscriber IdentityModule (USIM) may be used.

The UE selects a cell that has the greatest value and that belongs tocells having measured BS and signal intensity or quality greater than aspecific value (cell selection) (S420). In this case, the UE that ispowered off performs cell selection, which may be called initial cellselection. A cell selection procedure is described later in detail.After the cell selection, the UE receives system informationperiodically by the BS. The specific value refers to a value that isdefined in a system in order for the quality of a physical signal indata transmission/reception to be guaranteed. Accordingly, the specificvalue may differ depending on applied RAT.

If network registration is necessary, the UE performs a networkregistration procedure (S430). The UE registers its information (e.g.,an IMSI) with the network in order to receive service (e.g., paging)from the network. The UE does not register it with a network whenever itselects a cell, but registers it with a network when information aboutthe network (e.g., a Tracking Area Identity (TAI)) included in systeminformation is different from information about the network that isknown to the UE.

The UE performs cell reselection based on a service environment providedby the cell or the environment of the UE (S440). If the value of theintensity or quality of a signal measured based on a BS from which theUE is provided with service is lower than that measured based on a BS ofa neighboring cell, the UE selects a cell that belongs to other cellsand that provides better signal characteristics than the cell of the BSthat is accessed by the UE. This procedure is called cell reselectiondifferently from the initial cell selection of the No. 2 procedure. Inthis case, temporal restriction conditions are placed in order for acell to be frequently reselected in response to a change of signalcharacteristic. A cell reselection procedure is described later indetail.

FIG. 5 is a flowchart illustrating a procedure of establishing RRCconnection.

UE sends an RRC connection request message that requests RRC connectionto a network (S510). The network sends an RRC connection establishmentmessage as a response to the RRC connection request (S520). Afterreceiving the RRC connection establishment message, the UE enters RRCconnected mode.

The UE sends an RRC connection establishment complete message used tocheck the successful completion of the RRC connection to the network(S530).

FIG. 6 is a flowchart illustrating an RRC connection reconfigurationprocedure. An RRC connection reconfiguration is used to modify RRCconnection. This is used to establish/modify/release RBs, performhandover, and set up/modify/release measurements.

A network sends an RRC connection reconfiguration message for modifyingRRC connection to UE (S610). As a response to the RRC connectionreconfiguration message, the UE sends an RRC connection reconfigurationcomplete message used to check the successful completion of the RRCconnection reconfiguration to the network (S620).

Hereinafter, a public land mobile network (PLMN) is described.

The PLMN is a network which is disposed and operated by a mobile networkoperator. Each mobile network operator operates one or more PLMNs. EachPLMN may be identified by a Mobile Country Code (MCC) and a MobileNetwork Code (MNC). PLMN information of a cell is included in systeminformation and broadcasted.

In PLMN selection, cell selection, and cell reselection, various typesof PLMNs may be considered by the terminal.

Home PLMN (HPLMN): PLMN having MCC and MNC matching with MCC and MNC ofa terminal IMSI.

Equivalent HPLMN (EHPLMN): PLMN serving as an equivalent of an HPLMN.

Registered PLMN (RPLMN): PLMN successfully finishing locationregistration.

Equivalent PLMN (EPLMN): PLMN serving as an equivalent of an RPLMN.

Each mobile service consumer subscribes in the HPLMN. When a generalservice is provided to the terminal through the HPLMN or the EHPLMN, theterminal is not in a roaming state. Meanwhile, when the service isprovided to the terminal through a PLMN except for the HPLMN/EHPLMN, theterminal is in the roaming state. In this case, the PLMN refers to aVisited PLMN (VPLMN).

When UE is initially powered on, the UE searches for available PublicLand Mobile Networks (PLMNs) and selects a proper PLMN from which the UEis able to be provided with service. The PLMN is a network that isdeployed or operated by a mobile network operator. Each mobile networkoperator operates one or more PLMNs. Each PLMN may be identified byMobile Country Code (MCC) and Mobile Network Code (MNC). Informationabout the PLMN of a cell is included in system information andbroadcasted. The UE attempts to register it with the selected PLMN. Ifregistration is successful, the selected PLMN becomes a Registered PLMN(RPLMN). The network may signalize a PLMN list to the UE. In this case,PLMNs included in the PLMN list may be considered to be PLMNs, such asRPLMNs. The UE registered with the network needs to be able to be alwaysreachable by the network. If the UE is in the ECM-CONNECTED state(identically the RRC connection state), the network recognizes that theUE is being provided with service. If the UE is in the ECM-IDLE state(identically the RRC idle state), however, the situation of the UE isnot valid in an eNB, but is stored in the MME. In such a case, only theMME is informed of the location of the UE in the ECM-IDLE state throughthe granularity of the list of Tracking Areas (TAs). A single TA isidentified by a Tracking Area Identity (TAI) formed of the identifier ofa PLMN to which the TA belongs and Tracking Area Code (TAC) thatuniquely expresses the TA within the PLMN.

Thereafter, the UE selects a cell that belongs to cells provided by theselected PLMN and that has signal quality and characteristics on whichthe UE is able to be provided with proper service.

The following is a detailed description of a procedure of selecting acell by a terminal.

When power is turned-on or the terminal is located in a cell, theterminal performs procedures for receiving a service byselecting/reselecting a suitable quality cell.

A terminal in an RRC idle state should prepare to receive a servicethrough the cell by always selecting a suitable quality cell. Forexample, a terminal where power is turned-on just before should selectthe suitable quality cell to be registered in a network. If the terminalin an RRC connection state enters in an RRC idle state, the terminalshould selects a cell for stay in the RRC idle state. In this way, aprocedure of selecting a cell satisfying a certain condition by theterminal in order to be in a service idle state such as the RRC idlestate refers to cell selection. Since the cell selection is performed ina state that a cell in the RRC idle state is not currently determined,it is important to select the cell as rapid as possible. Accordingly, ifthe cell provides a wireless signal quality of a predetermined level orgreater, although the cell does not provide the best wireless signalquality, the cell may be selected during a cell selection procedure ofthe terminal.

A method and a procedure of selecting a cell by a terminal in a 3GPP LTEis described with reference to 3GPP TS 36.304 V8.5.0 (2009-03) “UserEquipment (UE) procedures in idle mode (Release 8)”.

A cell selection procedure is basically divided into two types.

The first is an initial cell selection procedure. In this procedure, UEdoes not have preliminary information about a wireless channel.Accordingly, the UE searches for all wireless channels in order to findout a proper cell. The UE searches for the strongest cell in eachchannel. Thereafter, if the UE has only to search for a suitable cellthat satisfies a cell selection criterion, the UE selects thecorresponding cell.

Next, the UE may select the cell using stored information or usinginformation broadcasted by the cell. Accordingly, cell selection may befast compared to an initial cell selection procedure. If the UE has onlyto search for a cell that satisfies the cell selection criterion, the UEselects the corresponding cell. If a suitable cell that satisfies thecell selection criterion is not retrieved though such a procedure, theUE performs an initial cell selection procedure.

A cell selection criterion may be defined as in Equation 1 below.Srxlev>0 AND Squal>0,whereSrxlev=Q _(rxlevmeas)−(Q _(rxlevmin) +Q _(rxlevminoffset))−P_(compensation),Squal=Q _(qualmeas)−(Q _(qualmin) +Q _(qualminoffset))  [Equation 1]

In this case, in Equation 1, the variables may be defined as in Table 1below.

TABLE 1 Srxlev Cell selection RX level value (dB) Squal Cell selectionquality value (dB) Q_(rxlevmeas) Measured cell RX level value (RSRP)Q_(qualmeas) Measured cell quality value (RSRQ) Q_(rxlevmin) Minimumrequired RX level in the cell (dBm) Q_(qualmin) Minimum required qualitylevel in the cell (dB) Q_(rxlevminoffset) Offset to the signalledQ_(rxlevmin) taken into account in the Srxlev evaluation as a result ofa periodic search for a higher priority PLMN while camped normally in aVPLMN Q_(qualminoffset) Offset to the signalled Q_(qualmin) taken intoaccount in the Squal evaluation as a result of a periodic search for ahigher priority PLMN while camped normally in a VPLMN Pcompensationmax(P_(EMAX) − P_(PowerClass), 0) (dB) P_(EMAX) Maximum TX power levelan UE may use when transmitting on the uplink in the cell (dBm) definedas P_(EMAX) in [TS 36.101] P_(PowerClass) Maximum RF output power of theUE (dBm) according to the UE power class as defined in [TS 36.101]

Qrxlevminoffset and Qqualminoffset, that is, signaled values, are theresults of periodic discovery for a PLMN having higher priority while UEcamps on a normal cell within a VPLMN, and may be applied only when cellselection is evaluated. As described above, during the periodicdiscovery of a PLMN having higher priority, UE may perform cellselection evaluation using parameter values stored from another cell ofthe PLMN having such higher priority.

After UE selects any cell through a cell selection procedure, theintensity or quality of a signal between the UE and a BS may be changeddue to the mobility of the UE or a change of a radio environment.Accordingly, if the quality of the selected cell is changed, the UE mayselect another cell providing better quality.

After the UE selects a specific cell through the cell selectionprocedure, the intensity or quality of a signal between the UE and a BSmay be changed due to a change in the mobility or wireless environmentof the UE. Accordingly, if the quality of the selected cell isdeteriorated, the UE may select another cell that provides betterquality. If a cell is reselected as described above, the UE selects acell that provides better signal quality than the currently selectedcell. Such a procedure is called cell reselection. In general, a basicobject of the cell reselection procedure is to select a cell thatprovides UE with the best quality from a viewpoint of the quality of aradio signal.

In addition to the viewpoint of the quality of a radio signal, a networkmay determine priority corresponding to each frequency, and may informthe UE of the determined priorities. The UE that has received thepriorities preferentially takes into consideration the priorities in acell reselection procedure compared to a radio signal quality criterion.

As described above, there is a method of selecting or reselecting a cellaccording to the signal characteristics of a wireless environment. Inselecting a cell for reselection when a cell is reselected, thefollowing cell reselection methods may be present according to the RATand frequency characteristics of the cell.

-   -   Intra-frequency cell reselection: UE reselects a cell having the        same center frequency as that of RAT, such as a cell on which        the UE camps on.    -   Inter-frequency cell reselection: UE reselects a cell having a        different center frequency from that of RAT, such as a cell on        which the UE camps on    -   Inter-RAT cell reselection: UE reselects a cell that uses RAT        different from RAT on which the UE camps

The principle of a cell reselection procedure is as follows.

First, UE measures the quality of a serving cell and neighbor cells forcell reselection.

Second, cell reselection is performed based on a cell reselectioncriterion. The cell reselection criterion has the followingcharacteristics in relation to the measurements of a serving cell andneighbor cells.

Intra-frequency cell reselection is basically based on ranking. Rankingis a task for defining a criterion value for evaluating cell reselectionand numbering cells using criterion values according to the size of thecriterion values. A cell having the best criterion is commonly calledthe best-ranked cell. The cell criterion value is based on the value ofa corresponding cell measured by UE, and may be a value to which afrequency offset or cell offset has been applied, if necessary.

Inter-frequency cell reselection is based on frequency priority providedby a network. UE attempts to camp on a frequency having the highestfrequency priority. A network may provide frequency priority that willbe applied by UEs within a cell in common through broadcastingsignaling, or may provide frequency-specific priority to each UE throughUE-dedicated signaling. A cell reselection priority provided throughbroadcast signaling may refer to a common priority. A cell reselectionpriority for each terminal set by a network may refer to a dedicatedpriority. If receiving the dedicated priority, the terminal may receivea valid time associated with the dedicated priority together. Ifreceiving the dedicated priority, the terminal starts a validity timerset as the received valid time together therewith. While the valid timeris operated, the terminal applies the dedicated priority in the RRC idlemode. If the valid timer is expired, the terminal discards the dedicatedpriority and again applies the common priority.

For the inter-frequency cell reselection, a network may provide UE witha parameter (e.g., a frequency-specific offset) used in cell reselectionfor each frequency. For the intra-frequency cell reselection or theinter-frequency cell reselection, a network may provide UE with aNeighboring Cell List (NCL) used in cell reselection. The NCL includes acell-specific parameter (e.g., a cell-specific offset) used in cellreselection. For the intra-frequency or inter-frequency cellreselection, a network may provide UE with a cell reselection black listused in cell reselection.

The UE does not perform cell reselection on a cell included in the blacklist.

Ranking performed in a cell reselection evaluation procedure isdescribed below.

A ranking criterion used to give the priority of a cell is defined as inEquation 2.R _(s) =Q _(meas,s) +Q _(hyst) , R _(n) =Q _(meas,n) −Q_(offset)  [Equation 2]

In Equation 2, Rs is the ranking criterion of a serving cell on which UEnow camps, Rn is the ranking criterion of a neighboring cell, Qmeas,s isthe quality value of the serving cell measured by the UE, Qmeas,n is thequality value of the neighboring cell measured by the UE, Qhyst is ahysteresis value for ranking, and Qoffset is an offset between the twocells.

In Intra-frequency, if UE receives an offset “Qoffsets,n” between aserving cell and a neighbor cell, Qoffset=Qoffsets,n. If UE does notQoffsets,n, Qoffset=0.

In Inter-frequency, if UE receives an offset “Qoffsets,n” for acorresponding cell, Qoffset=Qoffsets,n+Qfrequency. If UE does notreceive “Qoffsets,n”, Qoffset=Qfrequency.

If the ranking criterion Rs of a serving cell and the ranking criterionRn of a neighbor cell are changed in a similar state, ranking priorityis frequency changed as a result of the change, and UE may alternatelyreselect the twos. Qhyst is a parameter that gives hysteresis to cellreselection so that UE is prevented from to alternately reselecting twocells.

UE measures RS of a serving cell and Rn of a neighbor cell according tothe above equation, considers a cell having the greatest rankingcriterion value to be the best-ranked cell, and reselects the cell.

In accordance with the criterion, it may be checked that the quality ofa cell is the most important criterion in cell reselection. If areselected cell is not a suitable cell, UE excludes a correspondingfrequency or a corresponding cell from the subject of cell reselection.

Hereinafter, radio link failure (RLF) will be described.

UE continues to perform measurements in order to maintain the quality ofa radio link with a serving cell from which the UE receives service. TheUE determines whether or not communication is impossible in a currentsituation due to the deterioration of the quality of the radio link withthe serving cell. If communication is almost impossible because thequality of the serving cell is too low, the UE determines the currentsituation to be an RLF.

If the RLF is determined, the UE abandons maintaining communication withthe current serving cell, selects a new cell through cell selection (orcell reselection) procedure, and attempts RRC connectionre-establishment with the new cell.

In the specification of 3GPP LTE, the following examples are taken ascases where normal communication is impossible.

-   -   A case where UE determines that there is a serious problem in        the quality of a downlink communication link (a case where the        quality of a PCell is determined to be low while performing RLM)        based on the radio quality measured results of the PHY layer of        the UE    -   A case where uplink transmission is problematic because a random        access procedure continues to fail in the MAC sublayer.    -   A case where uplink transmission is problematic because uplink        data transmission continues to fail in the RLC sublayer.    -   A case where handover is determined to have failed.    -   A case where a message received by UE does not pass through an        integrity check.

An RRC connection re-establishment procedure is described in more detailbelow.

FIG. 7 is a diagram illustrating an RRC connection re-establishmentprocedure.

Referring to FIG. 7, UE stops using all the radio bearers that have beenconfigured other than a Signaling Radio Bearer (SRB) #0, and initializesa variety of kinds of sublayers of an Access Stratum (AS) (S710).Furthermore, the UE configures each sublayer and the PHY layer as adefault configuration. In this procedure, the UE maintains the RRCconnection state.

The UE performs a cell selection procedure for performing an RRCconnection reconfiguration procedure (S720). The cell selectionprocedure of the RRC connection re-establishment procedure may beperformed in the same manner as the cell selection procedure that isperformed by the UE in the RRC idle state, although the UE maintains theRRC connection state.

After performing the cell selection procedure, the UE determines whetheror not a corresponding cell is a suitable cell by checking the systeminformation of the corresponding cell (S730). If the selected cell isdetermined to be a suitable E-UTRAN cell, the UE sends an RRC connectionre-establishment request message to the corresponding cell (S740).

Meanwhile, if the selected cell is determined to be a cell that uses RATdifferent from that of the E-UTRAN through the cell selection procedurefor performing the RRC connection re-establishment procedure, the UEstops the RRC connection re-establishment procedure and enters the RRCidle state (S750).

The UE may be implemented to finish checking whether the selected cellis a suitable cell through the cell selection procedure and thereception of the system information of the selected cell. To this end,the UE may drive a timer when the RRC connection re-establishmentprocedure is started. The timer may be stopped if it is determined thatthe UE has selected a suitable cell. If the timer expires, the UE mayconsider that the RRC connection re-establishment procedure has failed,and may enter the RRC idle state. Such a timer is hereinafter called anRLF timer. In LTE spec TS 36.331, a timer named “T311” may be used as anRLF timer. The UE may obtain the set value of the timer from the systeminformation of the serving cell.

If an RRC connection re-establishment request message is received fromthe UE and the request is accepted, a cell sends an RRC connectionre-establishment message to the UE.

The UE that has received the RRC connection re-establishment messagefrom the cell reconfigures a PDCP sublayer and an RLC sublayer with anSRB1. Furthermore, the UE calculates various key values related tosecurity setting, and reconfigures a PDCP sublayer responsible forsecurity as the newly calculated security key values. Accordingly, theSRB 1 between the UE and the cell is open, and the UE and the cell mayexchange RRC control messages. The UE completes the restart of the SRB1,and sends an RRC connection re-establishment complete message indicativeof that the RRC connection re-establishment procedure has been completedto the cell (S760).

In contrast, if the RRC connection re-establishment request message isreceived from the UE and the request is not accepted, the cell sends anRRC connection re-establishment reject message to the UE.

If the RRC connection re-establishment procedure is successfullyperformed, the cell and the UE perform an RRC connection reconfigurationprocedure. Accordingly, the UE recovers the state prior to the executionof the RRC connection re-establishment procedure, and the continuity ofservice is guaranteed to the upmost.

FIG. 8 illustrates sub states where the terminal may have in an RRC_IDLEstate and a sub state transition process.

Referring to FIG. 8, a terminal performs an initial cell selectionprocess (S801). The initial cell selection process may be performed whenthere is no stored cell information with respect to the PLMN or asuitable cell is not found.

If the suitable cell is not found in the initial cell selection process,the terminal transitions to an any cell selection state (S802). Theoptional cell selection state represents a state which does not camp onin both of a suitable cell and an acceptable cell. The optional cellselection state is a state attempted by the terminal in order to find anacceptable cell of an optional PLMN which may camp on. When the terminalfinds no cells which may camp on, the terminal is continuouslymaintained in an optional cell selection state until the acceptable cellis found.

If the suitable cell is found in the initial cell selection process, thestate transits to a normal camp state (S803). The normal camp staterepresents a state which camps on the normal cell. A paging channel isselected according to information given through system information tomotor, and an evaluation process for cell reselection may be performed.

In the normal camp state (S803), if a cell reselection evaluationprocess (S804) is caused, the cell reselection evaluation process (S804)is performed. If a suitable cell is found in the cell reselectionevaluation process (S804), the terminal again transits to the normalcamp state (S803).

If an acceptable cell is found in the any cell selection state (S802),the terminal transits to an any cell camped state (S805). The any cellcamped state (S805) represents a state of camping on an acceptable cell.

In the any cell camped state (S805), the terminal may select a pagingchannel according to information given through system information tomonitor, and may perform a cell reselection evaluation process (S806).If the acceptable cell is not found in the cell reselection evaluationprocess (S806), the terminal transits the any cell selection state(S802).

Hereinafter, a D2D operation will be described. In the 3GPP LTE-A, aservice related to the D2D operation refers to Proximity based Services(ProSe). Hereinafter, the ProSe is an equivalent concept with the D2Doperation and the ProSe may be compatibly used with the D2D operation.The ProSe is now described.

The ProSe includes ProSe direct communication and ProSe directdiscovery. The ProSe direct communication presents communicationperformed by two or more adjacent terminals. The terminals may performcommunication using a protocol of a user plane. A ProSe-enabled UE meansa UE for supporting a process related to requirements of the ProSe.Unless otherwise defined, the ProSe-enabled UE includes both of a publicsafety UE and a non-public safety UE. The public safety UE represents aUE for supporting both of a public safety specified function and theProSe process. The non-public safety UE is a terminal which supports theProSe process but does not support the public safety specified function.

The ProSe direct discovery is a process where the ProSe-enabled UEdiscovers another ProSe-enabled UE. In this case, only ability of thetwo ProSe-enabled UEs is used. An EPC-level ProSe discovery signifies aprocess where an EPC determines whether 2 ProSe enable terminals areclosed to each other, and reports the close state thereof the two ProSeenabled terminals.

Hereinafter, the ProSe direct communication may refer to D2Dcommunication, and the ProSe direct discovery may refer to D2Ddiscovery.

FIG. 9 illustrates a reference structure for a ProSe.

Referring to FIG. 9, the reference structure for a ProSe includes aplurality of terminals having E-UTRAN, EPC, and ProSe applicationprogram, a ProSe application (APP) server, and a ProSe function.

An EPC is a representative example of the E-UTRAN. The EPC may includean MME, an S-GW, a P-GW, a policy and charging rules function (PCRF),and a home subscriber server (HSS).

The ProSe application server is a user of ProSe in order to make anapplication function. The ProSe application server may communicate withan application program in the terminal. The application program in theterminal may use a ProSe ability to make an application function.

The ProSe function may include at least one of following functions butis not limited thereto.

-   -   Interworking via a reference point towards the 3rd party        applications    -   Authorization and configuration of the UE for discovery and        direct communication)    -   Enable the function of the EPC level ProSe discovery    -   ProSe related new subscriber data and handling of data storage,        and also handling of ProSe identities    -   Security related function    -   Provide control towards the EPC for policy related function    -   Provide function for charging (via or outside of EPC, e.g.,        offline charging))

Hereinafter, a reference point and a reference interface will bedescribed in a reference structure for the ProSe.

-   -   PC1: a reference point between a ProSe application program in        the terminal and a ProSe application program in a ProSe        application server. The PC1 is used to define signaling        requirements in an application level.    -   PC2: is a reference point between the ProSe application server        and a ProSe function. The PC2 is used to define an interaction        between the ProSe application server and a ProSe function. An        application data update of a ProSe database of the ProSe        function may be an example of the interaction.    -   PC3: is a reference point between the terminal and the ProSe        function. The PC3 is used to define an interaction between the        terminal and the ProSe function. Configuration for ProSe        discovery and communication may be an example of the        interaction.    -   PC4: is a reference point between an EPC and the ProSe function.        The PC4 is used to define an interaction between the EPC and the        ProSe function. The interaction lay illustrate when a path for        1:1 communication or a ProSe service for real time session        management or mobility management are authorized.    -   PC5: is a reference point to use control/user plane for        discovery, communication, and relay between terminals, and 1:1        communication.    -   PC6: is a reference point to use a function such as ProSe        discovery between users included in different PLMNs.    -   SGi: may be used for application data and application level        control information exchange.

<ProSe Direct Communication (D2D Communication)>.

The ProSe direct communication is a communication mode where two publicsafety terminals may perform direct communication through a PC 5interface. The communication mode may be supported in both of a case ofreceiving a service in coverage of E-UTRAN or a case of separating thecoverage of E-UTRAN.

FIG. 10 illustrates arrangement examples of terminals performing ProSedirect communication and cell coverage.

Referring to FIG. 10(a), UEs A and B may be located outside of the cellcoverage. Referring to FIG. 10(b), the UE A may be located in the cellcoverage and the UE B may be located outside of the cell coverage.Referring to FIG. 10(c), both of UEs A and B may be located in the cellcoverage. Referring to FIG. 10(d), the UE A may be located in coverageof a first cell and the UE B may be in coverage of a second cell.

As described above, the ProSe direct communication may be performedbetween terminals which are provided at various positions.

Meanwhile, following IDs may be used in the ProSe direct communication.

Source layer-2 ID: The source layer-2 ID identifies a sender of a packetin a PC 5 interface.

Purpose layer-2 ID: The purpose layer-2 ID identifies a target of apacket in a PC 5 interface.

SA L1 ID: The SA L1 ID represents an in an ID in a scheduling assignment(SA) in the PC 5 interface.

FIG. 11 illustrates a user plane protocol stack for the ProSe directcommunication.

Referring to FIG. 11, the PC 5 interface includes a PDCH layer, a RLClayer, a MAC layer, and a PHY layer.

There may not be HARQ feedback in the ProSe direct communication. An MACheader may include the source layer-2 ID and the purpose layer-2 ID.

<Radio Resource Assignment for ProSe Direct Communication>.

A ProSe enable terminal may use following two modes with respect toresource assignments for the ProSe direct communication.

1. Mode 1

The mode 2 is a mode for receiving scheduling a resource for the ProSedirect communication from a base station. The terminal should be in aRRC_CONNECTED state according to the mode 1 in order to transmit data.The terminal requests a transmission resource to the base station, andthe base station schedules a resource for scheduling assignment and datatransmission. The terminal may transmit a scheduling request to the basestation and may transmit a Buffer Status Report (ProSe BSR). The basestation has data which the terminal will perform the ProSe directcommunication and determines whether a resource for transmitting thedata is required.

2. Mode 2

The mode 2 is a mode for selecting a direct resource. The terminaldirectly selects a resource for the ProSe direct communication from aresource pool. The resource pool may be configured by a network or maybe previously determined.

Meanwhile, when the terminal includes a serving cell, that is, when theterminal is in an RRC_CONNECTED state with the base station or islocated in a specific cell in an RRC_IDLE state, the terminal isregarded to be in coverage of the base station.

If the terminal is located outside of the coverage, only the mode 2 isapplicable. If the terminal is located in the coverage, the mode 1 orthe mode 2 may be used according to setting of the base station.

If there are no exceptional conditions, only when the base station isconfigured, the terminal may change a mode from the mode 1 to the mode 2or from the mode 2 to the mode 1.

<ProSe Direct Discovery (D2D Discovery)>

The ProSe direct discovery represents a process used to discover whenthe ProSe enabled terminal discovers other neighboring ProSe enabledterminal and refers to D2D direction discovery or D2D discovery. In thiscase, an E-UTRA wireless signal through the PC 4 interface may be used.Hereinafter, information used for the ProSe direct discovery refers todiscovery information.

FIG. 12 illustrates a PC 5 interface for D2D discovery.

Referring to FIG. 12, the PC 5 interface includes an MAC layer, a PHYlayer, and a ProSe Protocol layer being an upper layer. Permission forannouncement and monitoring of discovery information is handled in theupper layer ProSe Protocol. Contents of discovery information aretransparent to an access stratum (AS). The ProSe Protocol allows onlyvalid discovery information to be transferred to the AS forannouncement.

An MAC layer receives discovery information from the upper layer ProSeProtocol. An IP layer is not used for transmitting the discoveryinformation. The MAC layer determines a resource used in order toannounce the discovery information received from the upper layer. TheMAC layer makes and sends a protocol data unit (MAC PDU) to a physicallayer. An MAC header is not added.

There are two types of resource assignments for announcing the discoveryinformation.

1. Type 1

The type 1 is a method assigned so that resources for announcing thediscovery information are not terminal-specific and the base stationprovides resource pool configuration for announcing the discoveryinformation to the terminals. The configuration may be included in asystem information block (SIB) to be signaled in a broadcast scheme.Alternatively, the configuration may be included in a terminal specificRRC message to be provided. Alternatively, the configuration may bebroadcast-signaled or terminal-specific signaled of a different layerfrom the RRC message.

The terminal selects a resource from an indicated resource pool toannounce discovery information using the selected resource. The terminalmay announce discovery information through a resource optionallyselected during each discovery period.

2. Type 2

The type 2 is a method where resources for announcing the discoveryinformation are terminal-specifically assigned. A terminal in aRRC_CONNECTED state may request a resource for announcing a discoverysignal to the base station through a RRC signal. The base station mayassign a resource for announcing a discovery signal as an RRC signal. Aresource for monitoring the discovery signal in a configured resourcepool may be assigned in terminals.

With respect to a terminal in an RRC_IDLE state, a base station mayreport a type 1 resource pool for announcing the discovery signal as anSIB. Terminals where ProSe direct discovery is allowed use a type 1resource pool for announcing the discovery information in the RRC_IDLEstate. Alternatively, the base station 2) reports that the base stationsupports the ProSe direct discovery through the SIB but may not providethe resource for announcing the discovery information. In this case, theterminal should enter the RRC_CONNECTED state for announcing thediscovery information.

With respect to a terminal in an RRC_CONNECTED state, the base stationmay configure whether to use a type 1 resource pool or a type 2 resourcepool for announcing the discovery information through a RRC signal.

FIG. 13 illustrates an embodiment of a ProSe direct discovery procedure.

Referring to FIG. 13, it is assumed in a terminal A and a terminal Bthat a ProSe-enabled application program is operated, and the terminal Aand the terminal B are configured in a friend relationship to eachother, that is, a relationship capable of allowing D2D communicationwith each other in the application program. Hereinafter, the terminal Bmay be expressed as a friend of the terminal A. For example, theapplication program may be a social networking program. 3GPP Layerscorrespond to functions of an application program for using a ProSediscovery service regulated according to 3GPP.

A ProSe direct discovery between the terminal A and the terminal B mayperform a following procedure.

1. First, the terminal A performs regular application-Layercommunication with an application server. The above communication isperformed based on Application programming interface (API).

2. A ProSe enabled application program of the terminal A receives a listof application layer IDs having a friend relationship. The applicationlayer ID may generally be in the form of a network access ID. Forexample, an application layer ID of the terminal A may have a form suchas adam@example.com.

3. A terminal A requests private expression codes for a user and privateexpression codes for a friend of the user.

4. 3GPP layers transmit an expression code request to a ProSe server.

5. The ProSe server map application layer IDs provided from an operatoror a third application server to private expression codes. For example,an application layer ID such as adam@example.com. The mapping may beperformed based on parameters (e.g., mapping algorithms, key values, andthe like) received from an application service of the network.

6. The ProSe server responds the obtained expression codes to the 3GPPlayers. The 3GPP layers report that expression codes with respect to therequested application layer are successively received to the ProSeenabled application program. Further, a mapping table between theapplication layer IDs and the expression codes are generated.

7. The ProSe enabled application program requests the 3GPP layers tostart the discovery procedure. That is, when one of friends is locatedclose to the terminal A and direct communication may be performed, theProSe enabled application program attempts the discovery. 3GPP layersannounce a private expression code of the terminal A (that is,“GTER543$#2FSJ67DFSF” which is a private expression code ofadam@example.com in the above example). In mapping of an applicationlayer ID of a corresponding application program and the privateexpression code, the mapping relationship may be known by the previouslyreceived friends, and the mapping may be performed.

8. It is assumed that the terminal B is operating the same ProSe enabledapplication program as that of the terminal A, and the above steps 3 to6 may be executed. 3GPP layers included in the terminal B may performProSe discovery.

9. When the terminal B receives the above announce from the terminal A,the terminal B determines whether the private expression code includedin the announce is known by the terminal B or is mapped to anapplication layer ID. As illustrated in step 8, since the terminal Bperforms steps 3 to 6, the terminal B knows a private expression codewith respect to the terminal A, mapping of the private expression codeto the application layer ID, and which is a corresponding applicationprogram. Accordingly, the terminal B may discover the terminal B fromthe announce of the terminal A. The 3GPP layers in the terminal Bannounces that adam@example.com is discovered to the ProSe enableapplication program.

FIG. 13 illustrates a discovery procedure by taking into considerationthe terminals A and B, the ProSe server, and the application server.Only an operation side between the terminals A and B is described. Theterminal A transmits a signal called the announce (the procedure mayrefer to announcement), and the terminal B receives the announce todiscover the terminal A. That is, a discovery procedure of FIG. 13 in anoperation directly related to another terminal among operationsperformed by each terminal may refer to a single step discoveryprocedure may refer to a single step discovery procedure in a side ofone step.

FIG. 14 illustrates another embodiment of a ProSe direct discoveryprocedure.

In FIG. 14, it is assumed that the terminal 1 to the terminal 4 may beincluded in a specific group communication system enablers (GCSE) group.It is assumed that the terminal 1 is a discoverer and terminals 2, 3,and 4 are a discoveree. A terminal 5 is a terminal regardless of adiscovery procedure.

The terminal 1 and the terminals 2 to 4 may perform a followingoperation in a discovery procedure.

First, the terminal 1 broadcasts a targeted discovery request message(hereinafter referred to ‘discovery request message’ or ‘M1’) in orderto discover whether an optional terminal included in the GCSE group islocated around the terminal 1. The targeted discovery request messagemay include a unique application program group ID or a layer-2 group IDof the specific GCSE group. Further, the targeted discovery requestmessage may include a unique ID of the terminal 1, that is, anapplication program private ID. The targeted discovery request messagemay be received by the terminals.

The terminal 5 transmits no response messages. The terminals 2, 3, and 4included in the GCSE group transmit a targeted discovery responsemessage (hereinafter referred to as a discovery response message or M2)as a response to the targeted discovery request message. The targeteddiscovery response message may include a unique application programprivate ID of a terminal transmitting the message.

An operation of terminals in a ProSe discovery procedure illustrated inFIG. 14 will be described. A discoverer (UE 1) transmits the targeteddiscovery request message, and receives a targeted discovery responsemessage being a response thereto. In addition, if a discoveree (e.g., UE2) receives the targeted discovery request message, the discovereetransmits a targeted discovery response message as a response thereto.Accordingly, each terminal performs an operation a second step. In theabove side, a ProSe discovery procedure of FIG. 14 may refer to adiscovery procedure.

In addition to the discovery procedure illustrated in FIG. 14, if theterminal 1 (discoverer) transmits a discovery confirm message(hereinafter may refer to M3) as a response to the targeted discoveryresponse message, this may refer to a third step discovery procedure.

The present invention is described below.

UE supporting a D2D operation may provide another network node (e.g.,another piece of UE or a BS) with a relay functionality.

A relay may be classified as a UE-NW relay and a UE-UE relay dependingon that UE provides a relay functionality between what network nodes.

FIG. 15 shows a UE-NW relay.

Referring to FIG. 15, UE 2 153 functions as a UE-NW relay. That is, theUE 2 153 is a network node that relays a network 151 and UE 1 152 placedoutside the coverage 154 of the network. In this case, the UE 2 153 maybe said to be the UE-NE relay. A D2D operation may be performed betweenthe pieces of UE 1 and 2 152 and 153, and the existing cellularcommunication may be performed between the UE 2 153 and the network 151.In FIG. 15, since the UE 1 152 is placed outside the coverage of thenetwork, it may not communicate with the network 151 if the UE 2 153does not provide a relay functionality. The UE-NW relay transmits andreceives data to and from the UE 1 through communication between piecesof UE, and transmits and receives data to and from the network throughcommon UE-network communication. The UE-NW relay transmits and receivesdata to and from the UE 1 communication between (a D2D operation)between pieces of UE, and transmits and receives data to and from thenetwork through common UE-network communication.

FIG. 16 shows a UE-UE relay.

Referring to FIG. 16, UE 2 163 functions as a UE-UE relay. That is, theUE 2 163 is a network node that relays between specific UE 1 162 and UE3 161 placed outside the coverage of the specific UE 162. In this case,the UE 2 163 may be said to be the UE-UE relay. In FIG. 16, since boththe pieces of UE 1 and 3 162 and 161 are placed outside the coverage,they may not communicate with each other if the UE 2 163 does notprovide a relay functionality. A D2D operation may be performed betweenthe pieces of UE 1 and 2 162 and 163 and between the pieces of UE 2 and3 163 and 161. The UE-UE relay transmits and receives data to and fromthe UE 1 through communication (a D2D operation) between pieces of UE,and transmits and receives data to and from the UE 3 throughcommunication (a D2D operation) between pieces of UE.

<Number of Hops of Network Node Providing Relay Functionality>

It is assumed that a network node providing a relay functionality is arelay node. In this case, the number of hops of the relay node may bedefined as the number of communication links between the relay node andanother network node, that is, the subject of relay.

FIG. 17 shows the number of hops of a UE-NW relay.

Referring to FIG. 17, UE 2 153 may be a network node providing a relayfunctionality. Another network node, that is, the subject of relay, withrespect to UE 1 152, may be a BS 151. In this case, the number of hopsof the UE 2 153 is 1. The reason for this is that the number ofcommunication links between the UE 2 153 and the BS 151 is 1. A networknode providing a relay functionality, such as the UE 2 153, may signalthe number of hops. Accordingly, the UE 1 152 may be aware of the numberof hops of the UE 2 153.

FIG. 18 shows the number of hops of another UE-NW relay.

Referring to FIG. 18, UE 2 163 is a network node providing a relayfunctionality. Another network node, that is, the subject of relay, withrespect to UE 1 162, may be a BS 164. In this case, the number of hopsof the UE 2 163 is 2. The reason for this is that the number ofcommunication links between the UE 2 163 and UE 3 161 is 1, the numberof communication links between the UE 3 161 and the BS 164 is 1, andthus a total number of communication links between the UE 2 163 and theBS 164 is 2.

As described above, UE supporting a D2D operation may provide a relayfunctionality between a network and another piece of UE, and may alsoprovide a relay functionality between other pieces of UE.

UE providing a relay functionality may report its own relay operationstatus to a BS or another piece of UE.

FIG. 19 shows a method for reporting a relay operation status, which isperformed by UE.

Referring to FIG. 19, UE receives configuration information about arelay operation status report (S210).

The UE evaluates a reporting event related to the relay operation statusreport (S220). If the reporting event is satisfied, the UE transmits itsown relay operation status information (S230).

The reporting method of FIG. 19 is described in detail.

Configuration information about a relay operation status report(hereinafter referred to as “configuration information) may be used toconfigure a reporting event and pieces of information that need to beincluded in a relay operation status report for UE.

That is, which contents have to be included when a relay operationstatus report is made, when the report is made, and how the report isreported may be configured in the UE based on configuration information.

In accordance with the configuration information, if a specific event issatisfied, the UE may report its own relay operation status. Such aspecific event is called a reporting event, for convenience sake. Whenat least one reporting event is configured, the UE evaluates whether thereporting event is satisfied.

The reporting event may be any one of the following events, for example.

TABLE 2 Event DESCRIPTION Event_R1 Determine that it attempts to startrelay operation Event_R2 Determine that relay operation has already beenstarted Event_R3 Determine that it attempts to stop an on-going relayoperation Event_R4 Determine that an on-going relay operation has beenstopped Event_R5 Determine that it attempts to resume a stopped relayoperation Event_R6 Determine that a relay operation has been resumedEvent_R7 Determine that another piece of UE that has to provide a relayoperation has been detected Event_R8 Determine that UE is no longer ableto detect the presence of another piece of UE providing a relayoperation

Each of the events of Table 2 is described in detail.

1. Event_R1: UE determines that it attempts to start relay operation.

The event is for allowing UE to report a need to start a relay operationto a network. When the report triggered by the event is received, thenetwork may transmit a command that permits a relay operation to the UE.Furthermore, the network may also provide pieces of configurationinformation about the relay operation to the UE. The pieces ofconfiguration information about the relay operation may indicate radioresources which may be used in the relay operation.

For example, UE 1 may measure a signal transmitted by an adjacentanother network node, and may determine that it needs to start a relayoperation based on a result of the measurement. If the UE 1 determinesto start the relay operation as described above, Event_R1 is satisfied.Accordingly, the UE 1 may transmit relay status information, providingnotification that it has determined to start the relay operation, to anetwork or another piece of UE.

2. Event_R2: UE determines that a relay operation has already beenstarted.

The event is for allowing UE to report that a relay operation hasalready been started to a network. When the report triggered by theevent is received, the network may be aware that the relay operation hasalready been started by the UE. Accordingly, the network may transmitconfiguration information to the UE in order to assist or control therelay operation.

The event may be configured if UE is permitted to autonomously start arelay operation. That is, the event may be configured if the UE ispermitted to autonomously start or stop the relay operation even withoutthe explicit command of the network. The network may previouslyconfigure the event in the UE through broadcasting or UE-dedicatedsignaling so that the UE is permitted to autonomously start the relayoperation. Alternatively, a specific type of UE is also possible whichis permitted to autonomously start a relay operation even without beingpreviously configured by a network with respect to the start of aUE-autonomous relay operation. Even in this case, the UE is not excludedto start or stop the relay operation in response to the explicit commandof the network.

3. Event_R3: UE determines that it attempts to stop an on-going relayoperation

4. Event_R4: UE determines that an on-going relay operation has beenstopped

5. Event_R5: UE determines that it attempts to resume a stopped relayoperation

The events are events related to the stop/resumption of the relayoperation of UE. If UE does not have a new radio resource configurationrelated to a relay operation when receiving a command indicative of theresumption of the relay operation from a network, the UE may reuse aradio resource configuration which was used in a previous relayoperation.

6. Event_(—) R6: UE determines that a relay operation has been resumed

The event may be configured if UE is permitted to autonomously resume arelay operation. In this case, the UE may be permitted to reuse a radioresource configuration that was used in a previous relay operation.

7. Event_R7: UE determines that another piece of UE that has to providea relay operation has been detected

For example, it is assumed that UE 1 has received a signal from UE 2,but has been aware that the UE 2 is placed outside the coverage of anetwork as a result of the measurement/decoding of the signal. In thiscase, the UE 1 may determine that another piece of UE (i.e., the UE 2)to which a relay operation has to be provided.

8. Event_R8: UE determines that UE is no longer able to detect thepresence of another piece of UE providing a relay operation

Meanwhile, pieces of information that need to be included in a relayoperation status report may also be configured in UE. For example, thepieces of information may be configured based on the configurationinformation about a relay status report. A relay operation status reporttriggered by the reporting event may include some or all of thefollowing pieces of information, for example.

1) Information indicative of a relay operation status, 2) the ID of UEtransmitting a relay operation status report, 3) a list of the IDs ofother pieces of UE which may use the UE making the relay operationstatus report as a relay, 4) a list of the IDs of other pieces of UE towhich the UE making the relay operation status report attempts toprovide a relay functionality, which may include 4-1) the ID of new UEif UE already provides a relay functionality to other pieces of UE, butthe new UE is detected and the new UE also attempts to provide a relayfunctionality and 4-2) the ID of new UE if UE does not provide a relayfunctionality to other pieces of UE and the new UE is detected and thenew UE attempts to provide the relay functionality, 5) an indicatorindicating whether there is any one of other piece of UE that will use arelay operation status report, 6) the ID of a group to which target UEattempting to provide a relay functionality belongs, and 7) a directcommunication cluster ID between pieces of UE to which target UEattempting to provide a relay functionality belongs

The information indicative of a relay operation status may beinformation that provides notification of any one of the followingstatuses: i) the start of a relay operation is required, ii) a relayoperation has been started, iii) the stop of a relay operation isrequired, iv) a relay operation has been stopped, v) the resumption of arelay operation is required, and vi) a relay operation has been resumed,for example.

Meanwhile, information about the reception target of a relay operationstatus report may also be configured in UE. For example, the ID (or IDlist) of a network node permitted to receive a relay operation statusreport may be configured in UE that will send the relay operation statusreport. In this case, a network node may be another piece of UE or a BS.If the network node is a BS, the ID may be a cell ID, such as a physicalcell ID, a layer 2 cell ID, or a global cell ID. If the network node isUE, the ID may be the ID of UE or a specific ID equivalent to the ID ofthe UE.

Alternatively, UE may make a relay operation status report to only anetwork node that has sent configuration information about the relayoperation status report. That is, if a BS 1 has sent configurationinformation about a relay operation status report to UE, the UE that hasreceived the configuration information transmits the corresponding relayoperation status report to only the BS 1.

Meanwhile, if a reporting event (or condition) has been satisfied,whether UE has to make a report how much/frequently may also beconfigured. For example, one of the following two methods may beconfigured.

1) One-time reporting: UE may be configured to make a relay operationstatus report once if a reporting event (condition) is satisfied.

2) Periodic reporting: UE may be configured to periodically make a relayoperation status report if a reporting event (condition) is satisfied.In periodic reporting, a reporting cycle must be set.

Different reporting events may be always related to other reportingconfigurations, and the reporting configuration may be configuredindependently of a reporting event configuration. In this case, therelation between the reporting configuration and the reporting eventconfiguration needs to be provided.

A configuration regarding a relay operation status report may bedynamically provided by a network. Alternatively, the configurationregarding a relay operation status report may have been previouslydetermined. The configuration regarding a relay operation status reportmay be an RAN level configuration. The RAN level configuration means aconfiguration provided from a BS to UE through an interface between theUE and the BS. For example, the configuration may mean a configurationprovided to UE through an RRC message. Such an RRC message is notbroadcasted, but may be unicasted or multicasted to specific UE or aspecific UE group. Alternatively, the configuration regarding a relayoperation status report may be a configuration exceeding an RAN level.For example, the configuration may be performed in an NAS level. In thiscase, an MME determines and configures the NAS level and provides it toUE. The configuration may also be performed through an OMA DM. In thiscase, the OMA DM may be determined and configured by the server of anetwork, and may be provided to UE through an OMA DM process.

Methods for reporting a relay operation status if UE autonomouslychanges the status of a relay operation and if UE receives an explicitcommand from a network are described below.

FIG. 20 shows a method for reporting a relay operation status accordingto an embodiment of the present invention. It is assumed that UE 1 mayautonomously change the status of a relay operation. That is, it isassumed that the UE 1 may autonomously start, stop, and resume the relayoperation.

Referring to FIG. 20, the UE 1 is provided with configurationinformation about a relay operation status report (S221). It is assumedthat for example, Event_R2, Event_R4, and Event_R6 have been configuredas reporting events based on the configuration information.

The UE 1 may detect UE 2 placed outside the coverage of an eNB throughmeasurement (S222). Furthermore, the UE 1 may detect that the UE 2 isplaced inside the coverage of the UE 1. The UE 2 may directly notify theUE 1 that it is placed inside the coverage of the UE 1 or may notify theUE 1 that it has detected a signal transmitted by the UE 1 (e.g., asynchronization signal transmitted by the UE 1 or a signal providingnotification of the relay functionality of the UE 1) so that the UE 1 isdirectly notified that the UE 2 is placed inside the coverage of the UE1.

The UE 1 determines to start a relay operation and executes the relayoperation (S223). The UE 1 may start a relay operation so that anotherpiece of UE (e.g., the UE 2) communicates with another piece of UE(e.g., UE 3) or the eNB.

In this case, the UE 1 has started the relay operation and as a result,may determine that Event_R2 has been satisfied. Accordingly, the UE 1reports that it has started the relay operation to the eNB (S224). Thatis, the UE 1 may transmit relay operation status information indicatingthat the relay operation has been started to the eNB.

After a lapse of a specific time, the UE 1 may detect that the UE 2 hasbeen placed outside the coverage of the UE 1 and placed inside thecoverage of the eNB (S225).

In this case, the UE 1 may determine that it no longer needs to providea relay functionality to the UE 2. Accordingly, the UE 1determines/executes the stop of the relay operation (S226). As a result,Event_R4 is satisfied.

The UE 1 reports that it has stopped the relay operation to the eNBbecause the predetermined reporting event Event_R4 has been satisfied(S227). In other words, the UE 1 transmits relay operation statusinformation indicating that the relay operation has been stopped to theeNB.

After a lapse of a specific time, the UE 1 may detect that the UE 2 hasbeen placed outside the coverage of the eNB again (S228). Furthermore,the UE 1 may also detect that the UE 2 has been placed inside thecoverage of the UE 1.

The UE 1 determines to resume a relay operation and executes therelation operation (S229).

Event_R6 is satisfied because the UE 1 has resumed the relay operation.The UE 1 reports that it has resumed the relay operation to the eNBbecause the predetermined reporting event has been satisfied (S230). Inother words, the UE 1 transmits relay operation status informationindicative of the resumption of the relay operation to the eNB.

The method of FIG. 20 has been performed on the assumption that the UE 1is able to autonomously start/stop/resume a relay operation or the UE 1is permitted to autonomously start/stop/resume a relay operation. If theUE 1 is unable to autonomously send/stop/resume a relay operation or theUE 1 is not permitted to autonomously send/stop/resume a relayoperation, the UE 1 may operate according to the following embodiment.

FIG. 21 shows a method for reporting a relay operation status accordingto another embodiment of the present invention. It is assumed that UE 1is able to start, stop, and resume a relay operation only when itexplicitly receives a command from a network.

Referring to FIG. 21, the UE 1 is provided with configurationinformation about a relay operation status report (S321). It is assumedthat for example, Event_R1, Event_R3, and Event_R5 have been set asreporting events based on the configuration information.

The UE 1 may detect that UE 2 has been placed outside the coverage ofthe UE 1 through measurement (S322). Furthermore, the UE 1 may detectthat the UE 2 is placed inside the coverage of the UE 1.

The UE 1 may determine that it needs to provide a relay functionalityfor communication between another piece of UE (e.g., the UE 2) and yetanother piece of UE (e.g., UE 3) or an eNB, and may start a relayoperation (S323˜S325). Specifically, the UE 1 determines to start therelay operation (A of S323). Accordingly, since Event_R1 is satisfied,the UE 1 reports that it attempts to start the relay operation to theeNB (S324). In response thereto, the eNB transmits a command thatpermits the start the relay operation to the UE 1 (S325). The UE 1 thathas received the explicit command starts the relay operation (D ofS323).

That is, in the method of FIG. 20, the UE 1 has been illustrated asautonomously determining the start of the relay operation and executingthe relay operation. In contrast, in the method of FIG. 21, the UE 1receives the explicit command from the eNB and then starts the relayoperation.

After a lapse of a specific time, the UE 1 may detect that the UE 2 isnot placed inside the coverage of the UE 1 and is simultaneously placedinside the coverage of the eNB (S326).

In this case, the UE 1 determines to stop the relay operation because itno longer needs to provide the relay operation to the UE 2, and performsa corresponding operation (S327˜S329). Specifically, the UE 1 determinesto stop the relay operation (A of S327). Accordingly, since Event_R3that is a reporting event is satisfied, the UE 1 transmits relayoperation status information, providing notification that it attempts tostop the relay operation, to the eNB (S328). In response thereto, theeNB transmits a command that permits the UE 1 to stop the relayoperation to the UE 1 (S329). The UE 1 that has received the explicitcommand stops the relay operation (D of S327).

That is, in the method of FIG. 20, the UE 1 has been illustrated asautonomously determining the stop of the relay operation and executingthe relay operation. In contrast, in the method of FIG. 21, the UE 1receives the explicit command from the eNB and then stops the relayoperation.

After a lapse of a specific time, the UE 1 may detect that the UE 2 isplaced outside the coverage of the eNB and is simultaneously placedinside the coverage of the UE 1 (S330).

Accordingly, the UE 1 determines to resume a relay operation andexecutes the resumption of the relay operation (S331˜S333).Specifically, the UE 1 internally determines the resumption of the relayoperation (A of S331). Accordingly, since Event_R5 that is a reportingevent is satisfied, the UE 1 transmits relay operation statusinformation, providing notification that it attempts to resume the relayoperation, to the eNB (S332). In response thereto, the eNB transmits acommand that permits the UE 1 to resume the relay operation to the UE 1(S333). The UE 1 that has received the explicit command resumes therelay operation (D of S331).

That is, in the method of FIG. 20, the UE 1 has been illustrated asautonomously determining the resumption of the relay operation andexecuting the relay operation. In contrast, in the method of FIG. 21,the UE 1 receives the explicit command from the eNB and then resumes therelay operation.

FIG. 22 is a modified example of the embodiment of FIG. 21.

Referring to FIG. 22, UE 1 receives configuration information about arelay operation status report from an eNB (S421). The configurationinformation about the relay operation status report may include areporting event. For example, what the UE 1 detects another piece of UEand attempts to provide a relay functionality to the detected UE maybecome the reporting event.

The UE 1 detects UE 2 (S422). The UE 1 may detect the UE 2 by receivinga D2D discovery signal transmitted by the UE 2. Alternatively, the UE 1may detect the UE 2 by receiving D2D-related control informationtransmitted by the UE 2. In this case, the control information may havebeen broadcasted by the UE 2.

The UE 1 determines whether it may provide a relay functionality to thedetected UE (the UE 2) (S423).

If the UE 1 attempts to provide a relay functionality to the UE 2, itreports such an attempt to the eNB (S424). In this case, the UE 1 maynotify the eNB of the ID of the UE 2.

The eNB transmits configuration information that is required to providethe relay functionality to the UE 2 to the UE 1 (S425). For example, theeNB may notify the UE 1 what the relay functionality has to be providedusing which resource through the configuration information.

The UE 1 provides the relay functionality to the UE 2 based on theconfiguration information required to provide the relay functionality(S426).

FIG. 23 is a block diagram of UE in which the embodiment of the presentinvention is implemented.

Referring to FIG. 23, UE 1100 includes a processor 1110, memory 1120,and a radio frequency (RF) unit 1130. The processor 1110 implements theproposed functions, processes and/or methods. For example, the processor1110 may evaluate an event related to the relay operation status of theUE, and may transmit a report on the relay operation status of the UE ifthe event is satisfied.

The RF unit 1130 is connected to the processor 1110 and transmits andreceives radio signals.

The processor may include an application-specific integrated circuit(ASIC), other chipsets, logic circuits, and/or data processors. Thememory may include read-only memory (ROM), random access memory (RAM),flash memory, memory cards, storage media and/or other storage devices.The RF unit may include baseband circuits for processing radio signals.When an embodiment is implemented in software, the aforementioned schememay be implemented as a module (process, function or the like) thatperforms the aforementioned function. The module may be stored in thememory and executed by the processor. The memory may be placed inside oroutside the processor and may be connected to the processor by a varietyof well-known means.

What is claimed is:
 1. A method for transmitting a report for a relayoperation status in a wireless communication system, the methodperformed by a first user equipment (UE) and comprising: receiving, froma base station, information related with configuring the first UE toallow autonomously starting, stopping and resuming a relay operation;determining a second UE is outside coverages of the base station;transmitting, to the base station, the report for the relay operationstatus indicating that the relay operation has already been started,when the relay operation has already been started and the first UE isconfigured to allow autonomously starting the relay operation;determining the second UE has been placed outside coverage of the firstUE and placed inside coverage of the base station; transmitting, to thebase station, the report for the relay operation status indicating thatthe relay operation has already been stopped, when the relay operationhas already been stopped and the first UE is configured to allowautonomously stopping the relay operation; determining the second UE hasbeen placed outside coverage of the base station; and transmitting, tothe base station, the report for the relay operation status indicatingthat the relay operation has already been resumed, when the relayoperation has already been resumed and the first UE is configured toallow autonomously resuming the relay operation.
 2. The method of claim1, wherein the report for the relay operation comprises at least one ofstatus information indicative of the relay operation status and anidentity (ID) of the first UE and an ID of a subject of reception of thereport.
 3. The method of claim 1, wherein the first UE determines thatthe second UE has been detected when a device-to-device (D2D) discoverysignal is received from the second UE or control information related toD2D is received.
 4. The method of claim 1, wherein when the second UE isdetected, the first UE notifies the base station that the first UEattempts to provide a relay functionality to the second UE and receivesconfiguration information which permits a provision of the relayfunctionality from the base station.
 5. The method of claim 4, whereinthe first UE notifies the base station of an identity (ID) of the secondUE.
 6. The method of claim 4, wherein the configuration informationwhich permits the provision of the relay functionality comprises anidentity (ID) of the second UE.
 7. A first user equipment (UE),comprising: a radio frequency (RF) transceiver transmitting andreceiving radio signals; and a processor operating in conjunction withthe RF transceiver, wherein the processor is configured to: receive,from a base station, information related with configuring the first UEto allow autonomously starting, stopping and resuming a relay operation,determine a second UE is outside coverage of the base station, transmit,to the base station, the report for the relay operation statusindicating that the relay operation has already been started, when therelay operation has already been started and the first UE is configuredto allow autonomously starting the relay operation, determine the secondUE has been placed outside coverage of the first UE and placed insidecoverage of the base station, transmit, to the base station, the reportfor the relay operation status indicating that the relay operation hasalready been stopped, when the relay operation has already been stoppedand the first UE is configured to allow autonomously stopping the relayoperation, determine the second UE has been placed outside coverage ofthe base station, and transmit, to the base station, the report for therelay operation status indicating that the relay operation has alreadybeen resumed, when the relay operation has already been resumed and thefirst UE is configured to allow autonomously resuming the relayoperation.